Anal. Chem. 1990, 62, 141-146 (8) Ewina. A. G.; Wallinaford. R. A,; Olefirowicz, T. M. Anal. Chem. 1988, 67, 192A-303A. Cohen, A. S.;Najarian, D. R.; Paulus, A.; Guttman, A,; Smith, J. A,; Karger, E. L. Roc. Natl. Acad. Sci. USA 19889 85, 9660-9663. Guttman, A.; Paulus, A.; Cohen, A. S.;Grinberg, N.; Karger, B. L. J. Chromatogr. 1988, 448, 41-53. Electrophoresis '88-Proceedings of the 6th Meeting of the Interna tional Electrophoreski Society; Schaefer-Nieisen, C., Ed.; VCH Publishers: New York, 1988; pp 151-159. Cohen, A. S.; Karger, B. L. J. Chromatogr. 1987, 397,409-417. Hjerten, S.; Arch. Biochem. Biophys. Suppl. 1882, I , 147-151. Stellwagen, N. Advances in Nectrophoresis; Chrambach A,, Dunn, M. J., Radoia, E. J., Eds.; VCH Publishers: New York, 1987; Vol. 1, pp 179-228. . . - - - -. (15) Terabe, S.;Yashima, T.; Tanaka. N.; Araki, M. Anal. Chem. 1988, 60. 1673-1677. (16) Wieme, R. J. Chromatography: A Laboratory Handbook of Chromatography and Electrophoresis Methods, 3rd ed.;Heftmann, E., Ed.; Van
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Nostrand: New York, 1975; pp 228-281. (17) Nelson, R. J.; Cohen, A. S.;Paulus, A.; Guttman, A.; Karger, E. L. J. Chromatogr. 1888, 480, 111-127. (18) Foley, J. P.; Dorsey, J. G. Anal. Chem. 1983, 55, 730-737. (19) Lauer, H. H.; Maniglli, D. M. Anal. Chem. 1886, 58, 166-170. (20) Mlkkers, F. E. P.; Everaerts, F. M.; Verheggen, Th. P. E. M. J. Chromatogf. 1978, 169, 1-10, (21) Rose, D. J., Jr., Jorgenson, J. W. J. Chromatogr. 1888, 327,23-34. (22) Pentoney, S. L., Jr.: Zare, R. N.; Quint, J. F. Anal. Chem. 1989, 67, 1642- 1647.
RECEIVED for review July 3,1989. Accepted October 19,1989. The authors gratefully acknowledge Beckman Instruments for support of this work. This is Barnett Institute Contribution No. 382.
Sample Introduction Techniques for the Determination of Osmium Isotope Ratios by Inductively Coupled Plasma Mass Spectrometry D. Conrad Gregoire
Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, Canada K I A OE8
Reported are the relative merits of three sample introduction techniques for the determination of osmlum isotope ratios by inductively coupled plasma m a s spectrometry (ICP-MS). These are electrothermal vaporization ( E N ) , osmium tetraoxide vapor generation (VG), and conventional solution nebulization (SN). When Te was used as a matrix modifier, electrothermal vaporization was 3 orders of magnitude more sensitive (in terms of signal intenslty) for osmium determinations than were either solution nebulization or osmium tetraoxide vapor generation. The sensitivity of SN was increased by a factor of 30 by nebullzlng Os as the dissolved tetraoxide rather than in its reduced form. Osmium isotope ratios were determined with a precision of 0.2% relative standard deviation (RSD) by using solution nebulization, 0.3% by using vapor generation, and 0.5 % by using electrothermal vaporization.
In recent years, important new applications in the geosciences for osmium isotope data have emerged. Because 18'Re decays to lmOswith a relatively long half-life of approximately 40 billion years, this isotope pair is useful as a geochronometer ( I ) . For certain occurrences such as ultramafic intrusions, Re/Os dating is the only means available to the geologist for dating these rocks. Osmium isotope ratios have been used to calculate the accretion rate of extraterrestrial particles (2) and used as an indicator of crustal versus cosmic origin of platinum group metals in layers of sedimentary strata high in iridium content ( 3 ) ,especially those found a t the Cretaceous-Tertiary boundary ( 4 ) and other so-called mass extinction boundaries. Although the abundance of Os in the earth's crust is less than 1 part per billion (ppb), concentrations several orders of magnitude higher can be found in extraterrestrial and certain mantle-derived materials. Accurate and sensitive methods have been reported for the determination of Os at low levels, such as accelerator mass spectrometry (5),resonance
ionization mass spectrometry (6, 7), and ion sputtering mass spectrometry (8, 9). The principal disadvantages of these techniques are the low sample throughput available and the somewhat higher cost of the analytical instrumentation required. Inductively coupled plasma mass spectrometry (ICP-MS) is a relatively new analytical technique that has been applied to the determination of Os isotopes. The high temperature of the argon plasma coupled with atmospheric pressure sampling of the ionized sample by a quadrupole mass spectrometer makes it possible to quickly analyze solutions containing Os a t the parts-per-billion concentration level. An added advantage of ICP-MS over other techniques is the ability to interchange different modes of sample introduction, which can result in dramatic increases in analytical sensitivity as will be shown below. Osmium isotope ratios of iron meteorites and iridosmines were determined by Masuda et al. (10) by conventional solution nebulization ICP-MS using a concentric glass nebulizer, while Lichte et al. (11)determined the osmium isotope composition of a shale by using a glass frit nebulizer to increase the efficiency of sample transport to the plasma. An increase in integrated signal intensity of 100 times over that obtained with solution nebulization was reported by Russ et al. (12) when osmium was distilled as the tetraoxide vapor and transported directly into the argon plasma. Comparable results were obtained by Dickin et al. (13) using a similar osmium vapor generation technique. For the major osmium isotopes, a precision of 0.3% (13) and an accuracy of 0.1-0).2% were achieved when ICP-MS results were compared to those obtained by secondary ion mass spectrometry. However, relatively large absolute quantities of Os and hence large sample sizes would be required to accurately determine the ratio of 1870s to any other isotope because lE7Osgenerally accounts for less than 2% of the total quantity of Os present. Electrothermal vaporization techniques (ETV) were first coupled to ICP-MS by Park et al. (14, 15) and later applied to the determination of some platinum group elements by
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ICP-MS by Gregoire (16). Among the advantages of ETV techniques demonstrated by these authors are increased sensitivity, accurate measurement of isotope ratios, microliter sample volumes, and the optional use of matrix modification (16).
In this paper, a comparison is made of the accuracy, precision, and sensitivity with which Os isotope ratios can be determined by each of three sample introduction techniques: solution nebulization, tetraoxide vapor generation, and electrothermal vaporization.
EXPERIMENTAL SECTION Instrumentation. The ICP mass spectrometer used for this study was a Perkin-Elmer/Sciex Elan Model 250. An extended quartz torch was used throughout. For solution nebulization work, a Meinhard C3 concentric glass nebulizer was used and a sample uptake rate of 0.8 mL/min was controlled with a Gilson Minipuls 2 peristaltic pump. The osmium tetraoxide vapor generation apparatus was a batch type unit similar in construction to the apparatus used by Dickin et al. (13). The 250-mL reaction vessel was wrapped with heater tape and insulated with a layer of glass wool. Above the reaction vessel was mounted a condenser to remove excess water vapor from the carrier gas stream. Argon, used as the sparging/carrier gas, was introduced from the bottom of the reaction vessel a t a flow rate of 1.0 L/min. A 0.5-m, 5-mm4.d. length of Tygon or Pyrex glass tubing was used to transport the argon gas from the condenser to the torch. The electrothermal vaporization device, constructed in our laboratories, has been described elsewhere ( 1 4 , I 5 ) . The device consisted of a resistively heated strip of metal or a graphite platform measuring 11 X 4 mm. Sample material vaporized on the graphite surface was transported to the plasma by a stream of argon directed tangentially relative to the metal supports on which the graphite parts were mounted. A quartz cover, resembling an inverted thistle tube, formed an envelope above the vaporization surface and was designed to minimize condensation of sample material on the surface of the cover and to maximize the transport of analyte to the plasma. Sample vapor leaving the ETV unit was transported to the plasma via a length (0.5 m) of 5-mm-i.d. Tygon or Pyrex glass tubing. Four types of graphite were used as the vaporization substrate for ETV work crystalline graphite, pyrolytically coated crystalline graphite, solid anisotropic pyrolytic graphite, and glassy carbon. Graphite stock was obtained from Ultra Carbon (Bay City, MI) and machined in our laboratories. Preformed glassy carbon platforms (11x 4 x 1mm) were obtained from Ringsdorff (Bonn, FRG) and used without further machining. Tantalum metal (A. D. Mackay, Darien, CT) strips were cut from foil sheets (99.95% purity) 0.05 mm in thickness. A 3-mm-diameter steel ball bearing was used to impress a small concave depression in the center of the metal strip in order to contain the sample solution during the drying step. A specially designed double-pass spray chamber (to be described elsewhere) was used. This device allowed for switching from one sample introduction technique to another without the usual plasma shutdown. Ion lens settings and plasma conditions were optimized by using the steady-state signal produced by solution nebulization. This avoided the more cumbersome and lengthy procedures that would have been required had a transient signal such as that produced by using the ETV been used for optimization of instrument parameters. Reagents and Sample Preparation. All reagents used were of analytical reagent grade, and distilled deionized water was used throughout. Matrix modifier solutions were made by dissolving salts in distilled water, with the exception of Ni and Te, which were in solution as chlorides in 2 M HC1. Specpure (Aesar/ Johnson Matthey, Inc.) 1000 ppm Os as OsC1, in 5% HCl was used as the Os test solution for this study. Iridosmine samples, obtained from the Systematic Reference Series, National Mineral Collection of Canada, were fused in glassy carbon crucibles heated by a Meeker burner. Fifty milligrams of iridosmine (natural Ir/Os alloy) was mixed with 0.3 g of sodium peroxide and fused for a period of 5 min a t red heat. The cooled melt was dissolved in 10 mL of distilled water and later acifidified
Table I. Instrument Operating Parameters B lens
E l lens P lens S2 lens sampler skimmer
Mass Spectrometer Settings +2.4 V -16.4 V -14.2 V -10.7 V Ni, 1.14-mm orifice Ni, 0.89-mm orifice
Plasma Conditions (Solution Nebulization) 1.0 kW reflected power
